In digital and analogue communication systems such as wired, wireless and optical communication systems, receiver timing and transmitter timing must be properly synchronised in order for the receiver to correctly demodulate received signals. The degree of sensitivity to synchronisation varies between the different communications systems. An example of a communication system that is known to be sensitive to imprecise synchronisation is the orthogonal frequency division multiplexing (OFDM) based system.
OFDM is a signal modulation technique that enables efficient transmission of data over wireless or wired communication channels. This technique employs a concept of subcarrier frequencies in which the data being communicated over the channel is encoded into amplitude/phase relations of the subcarriers which are spaced apart by constant frequency differences.
OFDM has several advantages over conventional data communication methods. One advantage is high spectral efficiency. In OFDM, the subcarriers are mutually orthogonal so no frequency guard band between them is required and almost the whole bandwidth is dedicated to the data transmission. Another advantage is robustness against inter symbol interference (ISI). A so called guard time between OFDM symbols is used to eliminate ISI. In OFDM, a cyclic prefix, which is a copy of the last portion of the symbol appended to the front of itself or a copy of the symbol is appended to the back of itself, is usually transmitted prior to the OFDM symbol. The time guard interval is typically chosen to be longer than the longest channel delay spread. Yet another advantage is tolerance to frequency selective fading. If the number of sub-carrier frequency is chosen to be sufficiently large, the fading caused by multipath propagation can be considered to be flat for every sub-carrier frequency. This simplifies the equalisation process.
Although OFDM possesses these advantages, it also has some disadvantages. Amongst the most important ones is, as mentioned above, sensitivity to imprecise synchronisation.
If for example, the timing synchronisation in OFDM systems is not achieved it is not possible to remove the cyclic prefix at the correct time instances and, as a consequence, the fast Fourier transform (FFT) demodulator is supplied with incorrect OFDM symbols.
In the prior art, a number of solutions are known for timing synchronisation.
In the method described in the U.S. Pat. No. 7,039,000, a coarse index used to adjust an unsynchronised received OFDM signal is determined by correlating a first half of a training symbol sequence of the OFDM signal with a second half of the training symbol sequence, whereby a first correlation function is determined. This first correlation function is compared with a predetermined threshold to determine the coarse index (or coarse timing estimate). Then, in the coarsely adjusted OFDM signal, the first training symbol is correlated with the last training symbol giving rise to a second correlation function. The second correlation function is further compared with another predetermined threshold to determine a fine index. The fine index is used to adjust the coarsely adjusted OFDM signal so that the receiver is synchronised with the transmitted OFDM signal.
In the method described in the U.S. Pat. No. 5,732,113, two unique training symbols are used to achieve timing and frequency synchronisation. The unique training symbol halves are identical in time order by transmitting a pseudo-random (PN) sequence on the even frequencies, while zeros are utilized on the odd frequencies. The time synchronisation process relies on autocorrelation of the first training symbol (a criterion function). Then, it determines the timing estimate by comparing the criterion function with a predetermined threshold. For achieving frequency synchronisation, a second autocorrelation of the second training symbol is used.
A drawback with the methods described in the prior art described above is that these methods rely on simple criterion functions and/or correlation functions which can be auto- or crosscorrelation or both and that the synchronisation is deemed to occur where the criterion functions exceeds some predetermined threshold (or thresholds). This leads to increased uncertainty in timing estimation and reduces accuracy of the synchronisation because the location of a peak or peaks of the criterion function(s) may vary significantly due to the delay spread of the channel.
Another drawback with the prior art solutions described above concerns the possibility that criterion functions exceed the threshold(s) too early so that false synchronisations occur (false alarm). This is especially the case when the quality of the received signal is bad due to high noise level and severe multipath conditions. As a consequence the whole OFDM signal may be rejected by the receiver since the timing estimation is incorrect.